Various paper and similar products have been impregnated with polyisocyanates for uses which include decorative paneling and structural skins of products such as foam or honeycomb filled sandwich panels. Many other applications, such as air filters and floor and roof underlayments, have also become important. Hunter, in U.S. Pat. No. 5,008,359, describes one method of making such products. A sheeted cellulosic paper, such as kraf linerboard, is at least partially impregnated with an essentially uncatalyzed polyfunctional isocyanate. Normally several sheets so impregnated would be superposed and subsequently cured in a press under heat and considerable pressure to form thin panels. Use of any customary catalysts was avoided since they appeared to cause poor adhesion between various plies of the laminates. Preferred pressing temperatures are in excess of 150.degree. C. with pressures of 3000 kPa or greater. A poly(diphenylmethane diisocyanate) (PMDI) appeared to be preferred as the impregnant.
Hunter et al., in U.S. Pat. No. 5,140,086 describe an apparent improvement on the above process. In order to achieve better and more uniform impregnation of the cellulosic substrate the isocyanate is applied in admixture with a miscible organic solvent. Propylene carbonate is the preferred solvent. This was chosen because of its low toxicity, viscosity and vapor pressure at room temperature, its high boiling point of 242.degree. C., and because it is substantially odorless and colorless. A high boiling point material was desirable to prevent blistering during the heat curing operation. In addition to improving impregnation uniformity, propylene carbonate used in a range of 5-20%, gave improved physical properties. It was speculated that the propylene carbonate also may serve as a copolymerizable reactant to some extent. Curing of the product was done under conditions of heat and pressure similar to those described above. It was permissible to include up to 0.5% of a catalyst with the isocyanate-solvent mixture to accelerate the curing reaction.
A later patent to the same inventors, U.S. Pat. No. 5,280,097, is directed to making laminated panel materials in which the earlier products and methods are used as substrates for decorative overlays. The overlay, such as a melamine resin treated printed paper, could be applied to the isocyanate treated substrate or to a laminate formed from a number of substrate plies. These were then preferably cured simultaneously in a single pressing step.
Dimakis, in U.S. Pat. Nos. 5,220,760 and 5,345,738, describes foam filled structural panels made using skins of kraft paper impregnated as taught in one of the above patents.
All of the above noted methods and products require the use of extremely expensive presses for curing the product. However, it has been known that polyisocyanates impregnated into cellulosic substrates will cure to insoluble polymers over prolonged periods of time at ambient conditions. This is believed to be due to slow reaction with the natural moisture present in the substrate, with atmospheric moisture, and possibly by reaction with hydroxyl groups of the cellulosic substrate. PMDI is generally the isocyanate of choice. Wallick, in U.S. Pat. Nos. 5,292,391 and 5,332,458 teaches application of a material such as PMDI to corrugated medium for strength enhancement of corrugated container board without adversely affecting repulpability of the product. The preferred procedure is to apply the isocyanate by spraying after corrugation but prior to application of adhesive at the single facer of the corrugator. Heat curing is thereafter minimized to enhance repulpability. Curing of the isocyanate impregnant continued well after application of the second liner to the product.
PMDI products are composed of a wide range of oligomriers and varying amounts, typically 40-60% of monomeric 4,4'-diphenylmethane diisocyanate (MDI). NCO-- functionality of the products sold as PMDI can vary widely, e.g., from about 1.4 to 3.2, with viscosities ranging from about 50 to 1800 cps. As the curing reaction with water occurs under ambient conditions an intermediate reaction product is 4,4'-diaminodiphenylmethane (MDA). This, in turn, again reacts with available --NCO groups to ultimately form insoluble polyureas. MDA is a relatively toxic chemical and it is desirable that its content, as well as the content of residual unreacted MDI, should be as low as possible. Thus it is highly desirable that cure rates should be relatively rapid and that the curing reaction of the PMDI should approach completion with a minimum of unreacted products. If suitably rapid cure rates could be achieved under ambient conditions numerous applications of the polymer impregnated product would present themselves. In the case of flat panels, elimination of the hot pressing step would considerably reduce the cost. Park et al. in U.S. Pat. No. 5,580,922 describe a cellulose product impregnated with a polyisocyanate along with a smaller amount of triacetin or triethyl citrate. The additives speed curing of the isocyanate to polyureas under room temperature conditions and reduce the presence of undesirable reaction intermediate products.
While PMDI is used to a large extent for preparation of urethane foams and other products by reaction with various polyols or polyethers, it is also used by itself as an adhesive for medium density composite wood products and oriented strand boards. However, in all of these uses the product is hot pressed to effect the reaction to polyureas in a minimum time.
Certain applications of products such as those described above in the Hunter et al. or Park et al. patents would find broader application if they had enhanced resistance to burning. Urethane foams have long employed organophosphorous based or heavily halogenated chemicals for inducing fire resistance. For best results, these are frequently used in combination with an inorganic additive, typically antimony oxide. However, the chemistry of urethanes is quite different from that of the present system and it is unclear whether those fire retardants conventionally used with urethianes would be acceptable at all. Extensive experimental work has indeed shown this to be the case. Problems of coreactivity and lack of compatibility with PMDI have greatly complicated the problem. The present inventors have no knowledge of fire retardant materials being successfully used with neat polyisocyanates where they are not to be further reacted with other materials such as polyols.
The present invention presents a solution to the problem of making fire retardant PMDI impregnated cellulose products in its finding that a very limited group of additives are highly effective in conveying fire resistance.